Low-temperature observation of the softened C-H stretching vibrations of cyclohexane on Rh(111)

The C-H stretching vibrations of cyclohexane on Rh(111) were investigated using infrared reflection absorption spectroscopy between 20 and 89 K. At 20 K, the softened C-H stretching band consists of several sharp peaks, ranging from 2500 to $2700{\text{cm}}^{-1}$. The wide-range distribution of the softened C-H stretching peaks results from inhomogeneity of adsorption environments. With increasing the substrate temperature, each softened C-H stretching peak becomes significantly broadened, but the normal C-H stretching peaks are little changed. These results indicate that the local interaction between the softened C-H species and the Rh(111) surface is sensitive to a thermally excited low-energy mode. The temperature-dependent broadening of a soft mode at low temperature is analyzed using a vibrational dephasing model, where the softened C-H stretching mode is anharmonically coupled with a thermally excited frustrated translation mode.

Figure 1

(a)–(d) IRAS spectra of cyclohexane on Rh(111) at a near saturation coverage as a function of substrate temperature. (e) The IRAS spectrum of cyclohexane on H-saturated Rh(111) at 90 K.

Figure 2

(a) FWHMs and (b) peak positions of a softened C-H stretching peak at $2590{\text{cm}}^{-1}$ (closed circles) as a function of temperature. FWHMs of a normal C-H peak at $2894{\text{cm}}^{-1}$ are also shown (open circles). FWHMs and peak positions of the soft mode are fitted by the dephasing model (Ref. 23). The solid line is the result using values of ${\omega }_0=24{\text{cm}}^{-1}$, $\delta \omega =23{\text{cm}}^{-1}$, and $\eta =4.1{\text{cm}}^{-1}$, and the dashed line represents the result of ${\omega }_0=56{\text{cm}}^{-1}$, $\delta \omega =46{\text{cm}}^{-1}$, and $\eta =13{\text{cm}}^{-1}$.